Although fuel cell vehicles (FCEV) are now generally sold, a polymer electrolyte fuel cell (PEFC) that uses hydrogen gas as fuel is adopted in such FCEVs. In addition, business-use or home-use stationary electric power sources that adopt a solid oxide fuel cell (SOFC) are now also generally sold. The SOFC has a feature that its operational temperature is high but its efficiency is high. In such an SOFC system, its start-up takes much time in order to increase temperature of its fuel cell stacks (FC stacks) to its high operational temperature.
In order to adapt a solid oxide fuel cell (SOFC) to a mobile vehicle such as a fuel cell vehicle (FCEV), it is necessary to shorten its start-up time while getting a required output power. Therefore, a Patent Literature 1 listed below discloses an SOFC system that includes a small-sized FC stack and a large-sized FC stack. In this SOFC system, it is proposed that temperature of the small-sized FC stack is quickly increased to its operational temperature first to make the SOFC system operable and then temperature of the large-sized FC stack is also increased to its operational temperature to get stable output power. Note that shortening of a start-up time is advantageous not only for mobile vehicles but also for stationary electric power sources.